1. Field of the Invention
The present invention relates to a curing mold for vulcanizing green tires and thereby manufacturing pneumatic tires.
2. Description of Related Art
Conventionally, various types of curing molds for manufacturing pneumatic tires are known. For example, a so-called insert segment type curing mold includes a plurality of fitting pieces each having an inner surface with concave or convex portions corresponding to land or groove portions of a product tire at least in its tread portion. A set of fitting pieces is mutually adjoined with each other and secured to a supporting member to form a segment, and a set of segments is mutually adjoined with each other into an annular assembly with a predetermined inner surface configuration.
It is generally necessary during the vulcanization process to tightly urge a green tire against the inner surface of the mold by means of an expansible bladder. Thus, air within a space between the outer surface of the green tire and the inner surface of the mold has to be discharged via boundary portions 12 between adjacent fitting pieces 10, 10, as shown in FIG. 1A. On this occasion, part of non-vulcanized rubber tends to be extruded from the mold along the boundary portions 12. Consequently, the insert segment type curing mold suffers from a problem that undesirable burrs tend to be formed at the land portion of the product tire inclusive of the ground contact portion. Such burrs protrude radially outwardly of the product tire and significantly deteriorate the appearance of the tire.
For preventing formation of burrs at the land portion of the product tire, one possible solution may be to arrange the boundary portion 12 of the adjacent fitting pieces 10 at a location corresponding to a groove portion of the product tire or to a protrusion 14 on the inner surface of the mold, as shown in FIG. 1B. However, such a structure makes it difficult to properly discharge air from the inside space of the mold. Rather, air tends to remain at a portion of the mold space which corresponds to the junction of a land portion and an adjacent groove portion of the product tire. The residual air within the mold often forms so-called "sags" and "bubbles" along the edge of the land portion of the product tire, and significantly deteriorates the appearance of the tire.
For overcoming the abovementioned drawbacks, another type of curing mold is known such as a "backbone" type mold which is disclosed, for example, in Japanese Patent Examined Publication No. 1-51,324. This type of curing mold includes an annular mold main body with a basic inner surface configuration corresponding to the land portion of a product tire in at least a tread portion thereof, and a plurality of projections corresponding to the groove portions of the product tire which are detachably secured to the main body for realizing a mold of a desired inner surface configuration.
Such an arrangement of a "backbone" type curing mold proved to be advantageous in that air within the mold can be discharged from the gap between the main body and the projections, corresponding to the edge of the land portion of a product tire, without forming burrs at the ground contacting portion of the product tire. However, this type of curing mold suffers from different disadvantages as follows. At the outset the, fastening operation for securing the projections to the mold main body with a number of bolts or the like is time-consuming and troublesome. It is also difficult to uniformly and accurately secure the projections to the mold main body for improving the product quality in terms of uniformity and accuracy. Moreover, for ensuring a stable discharge of air from the mold through the gap between the mold main body and the projections, it is necessary to periodically separate the projections from the mold main body so that not only the assembly of the curing mold, but also its maintenance is troublesome.
Apart from the above, there may be instances wherein it is desirable to modify the existing curing mold in view of minor changes in design of the tread pattern. However, known curing molds are not always suitable for such modification, and it has been a general practice to prepare entirely new molds for respective tread patterns even in the case of minor changes.